I'm just curious. I've seen other peregrine org's also feed quail to their injured birds. What makes quail such a good meal source?
Lots of places use quail for injured food, if they can access it at reasonable rates. In fact, it is not unusual for falconers and breeding facilities to raise their own quail for their birds. In our case, the quail we access is raised for their eggs and meat and when its available, we can get quail for our peregrines. As for what is a reasonable rate, I would suppose that depends on the rehab organization and what percentage of their guests could use quail in their diet and if their finances make it feasible to purchase - keep in mind, they have similar purchasing decisions to make for all their other guests - some of whom may have equally interesting "preferences". And I should say its a preference because not all raptors eat birds, some are more mammal hunters so mice and rats are another choice - for peregrines to but they can often get fussy about it (or not depending on the bird).
In terms of maximizing recovery, having a food source that is as close to wild game as possible seems to make a difference - or at least we hope so. Birds in care often have pale feet, like chicks do, its a function of what they are eating. I don't know specifically what chemicals or compounds or ratio of this to that will do it (and likely someone has looked at it but I don't have that information) but it has always seemed to me that it is "wild" food - not just the eating of wild food but the wild acquisition of food. Peregrines in care often don't get to go through the motions of acquiring their food until they get closer to release. Peregrine chick feet are very pale when they are young, get a bit more yellow as they grow so by the time we band them they are more yellow but not nearly as golden yellow as their parents. The feet of birds in care often pale some while in care. Captive-bred chicks often had pale feet too and like injured birds released, they "yellow-up" after they fledge and get hunting. Birds that have been in care - chicks and adults - can have paler feet as well, but they too darken as they get better or as they shift into a captive life with balanced long-term meal plan that simulates their wild dietary needs. And quail, though raised, is more of a gamebird than chicken or turkey (excluding wild turkeys which are a whole other kettle of feathers). The quail we purchase come with feathers on them, (good for keeping her digestive system clean) are about the size of a pigeon which is the right size for a peregrine to munch on. Given that our goal is to help Beatrix recover, reducing her stress in care and maximizing the benefit of her calories is important - and if quail does that for her and we can access/afford if for her, then quail it is. A couple of other peregrines have had short stays this summer as well and they have benefited from our some access to Beatrix's larder as well. And it helps the rehab folks spend more of their money on food for their other patients/residents. Win-Win-Win.
Now all of that is just my observations over the years. Falconers, rehab folks and breeders have much more experience and would like have a great range of observations - some agreeing, some probably countering. As with all things, there are more shades of grey (or yellow in this case) the more access one has and my access in this particular area is not as great as many others so I'm just recounting what I have observed.
Hope that helps answer the question to some extent.